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Based on the technology of combustion synthesis in high-gravity field, the TiC-TiB2 ceramics with refined microstructures have been fabricated by adjusting the technological parameter and proportioning of raw starting powders. The effects of particle size of raw powders on crystal growth and mechanical properties were studied in this paper. It found that the propagation rate and the combustion temperature can be improved by decreasing the grain size of raw powders, as well as the heat exchange and mass diffusion were enhanced too, so that the microstructure homogenization, densification and mechanical properties of the TiC-TiB2 ceramics were improved accordingly. The highest density, relative density, Vickers hardness and fracture toughness of TiC-TiB2 composites measured 4.07 g/cm3, 87.5%, 22.5 ± 1.2 GPa and 9.5 ± 0.8 MPa·m0.5, respectively, since that the fine B4C powder with particle size < 3.5 µm and the fine Ti powder with particle size < 38 µm were chosed as raw starting powders. The fracture behaviour of TiC-TiB2 composites was dominated by the strong coupled toughening mechanisms of crack deflection and crack-bridging, and the high fracture toughness of the TiC-TiB2 ceramics benefits mainly from the achievement of micro-nanocrystalline microstructure in the full-density solidified ceramic.

By taking combustion synthesis in ultrahigh gravity field to prepare solidified TiC-TiB2 composite ceramic, laminated composite of the ceramic to Ti-6Al-4V was achieved by fusion bonding and atomic inter-diffusion, and within the joint there formed the unique microstructure of multiscale (micrometer-micro/nanometer-nanometer) and multilevel characterized by size and distribution of TiB2 and TiB due to the presence of a series of metallurgical reaction including peritectic reaction of solidified and Ti liquid, direct growth of TiB solids from liquid Ti and subsequent eutectic reaction of TiB solids and liquid Ti. FEMSEM images of crack propagation paths at the joint showed despite the contribution from crack bridging of micro-nanometer TiB2 and TiB platelets to crack propagation resistance, residual stress toughening and subsequent crack pinning by micro-nanometer TiB2 ,TiB platelets and needle-like nanometer TiB grains as well as ductile phase toughening and bridging toughening by Ti metallic phases became the primary resistances to crack propagation, thereby presenting the delayed fracture behavior in the joint, so that shear fracture usually occurred at the solidified Ti alloy rather than the joint, and shear strength of 375 ± 55 MPa was achieved between the solidified ceramic and Ti alloy.

By taking combustion synthesis in enhanced high gravity field with increasing acceleration from 500 g to 2500 g at intervals of 500 g, a series of TiC-66.7 mol%TiB2 composites were prepared through liquid forming and rapid solidification. The ceramics were composed of TiB2 primary phases, TiC secondary phases and a few of Al2O3 inclusion and Cr metallic phases. Increasing high gravity acceleration not only promoted the combustion mode transfer from steady SHS mode to explosive one through enhancing the deposition of liquid product toward unreacted blend, but also accelerated liquid-liquid separation of TiC-TiB2 / Al2O3 droplets and subsequent formation of layered melt through enhancing Stokes flow in mixed liquid products, thereby reducing sharply Al2O3 inclusions and shrinkage cavities in solidified TiC-TiB2 composite. Meanwhile, the enhanced Stokes flow brought about constitutional uniformity through enhancing atomic diffusion in TiC-TiB2 liquid, not only refining the microstructure of the solidified ceramic, but also improving the homogeneity of the ceramic by inducing TiB2 primary phases to grow at similar velocity. As a result, hardness, flexural strength and fracture toughness of the composite increased greatly in enhanced high-gravity field.

The solidified TiC-TiB2 ceramics with excessive Ti mole content were prepared through adjusting the Ti additional agent in combustion system with ultra-high-gravity of 2000 g, and the ceramics were comprised of TiB2 primary phases, irregular TiC1-x secondary phases, a few of Al2O3 inclusions and Cr-Ti-Al metallic phases. The additional Ti content resulted in not only the rapidly-reduced Al2O3 inclusions but also the refined microstructure and the improved homogeneity in the solidified microstructure. As the Ti additional agent was increased, more and more Ti atoms could participate in the formation of nonstoichiometric TiC1-x phases and Cr-Ti-Al metallic phases. The maximum fracture toughness of 18.5 ± 1.0 MP · m0.5 in current solidified TiC-TiB2composite was achieved because of the less Al2O3 inclusions, the refined microstructure and the Ti-rich Cr-Ti-Al metallic phases obtained in the near-full-density composite. The high fracture toughness contributed from not only mechanism of crack deflection, crack-bridging and pull-out by a large number of fine TiB2 platelets, but also the ductile fracture toughening result from plastic deformation of Cr-Ti-Al metallic phases.

Based on taking combustion synthesis in ultrahigh-gravity field to prepare solidified TiC-TiB2 ceramic, laminated composite of TiC-TiB2 to Ti-6Al-4V was successfully obtained by fusion bonding, and within the joint there achieved multi-scale (micron-submicron-micro-nano) and multi-level (TiC1-x-TiB-TiB2 + TiB2-Ti-TiC1-x-TiB + TiB2-TiC1-x-TiB-Ti + TiB-TiC1-x-Ti + TiC1-x-Ti + Ti) composite structures characterized by size and distribution of TiB2 and TiB phases. As micro-nano TiB2, TiB platelets and needle-like nano TiB grains at the joint area initiated residual stress toughening and pinning at crack tip, while Ti phases initiated ductile phase toughening and subsequent bridging in crack-tip wake, a typical delayed fracture under external stress was manifested obviously in the joint of TiC-TiB2 to Ti-6Al-4V. As a result, the shear fracture of the joint of ceramic toTi-6Al-4V alloy usually occurred in solidified area of Ti alloy rather than the joint, and the shear strength between the ceramic and Ti alloy was maintained at 375 ± 55 MPa.

Based on using combustion synthesis in high-gravity field to prepare the solidified TiC-TiB2 composites, the layered composites of the ceramic to Ti-6Al-4V at graded composition were achieved by fusion joint and atomic inter-diffusion between liquid TiC-TiB2 and Ti-6Al-4V substrate. The mechanical properties showed that the relative density, micro-hardness and fracture toughness of TiC-TiB2 ceramic layer measured 98.5%, 21.5 GPa and 13.5 ± 2.5 MPa • m0.5, respectively, and the shear strength at joint of TiC-TiB2 to Ti-6Al-4V measured 450 ± 25 MPa. By conducting DOP test to evaluate ballistic performance of the ceramic and layered-composite targets against long-rod KE projectiles at impact velocity of 1.4 km • s-1, it was obtained the mass efficiency 3.28 of the solidified TiC-TiB2 was achieved as the impact point of the projectile was nearby the centre of the target, whereas the mass efficiency 3.18 of the layered composite with the solidified ceramic to Ti-6Al-4V was also achieved even if the impact point of the projectile was at the edge of the target. By combining penetration damage of the targets with the dynamic behavior of the ceramic, it was obtained that the layered composite achieved by the joint of the solidified ceramic to Ti-6Al-4V not only improve ballistic performance of the ceramic, but also weaken the sensitivity of the ceramic target against the impact point of KE projectile by tearing the joint of the ceramic with Ti alloy to restrain formation and propagation of conical crack and fracture cone in the ceramic.

Based on fusion bonding of liquid TiC-TiB2 ceramic film and molten Ti-6Al-4V substrate to induce the surface alloying of Ti-6Al-4V substrate, the TiC-TiB2 ceramic coating on Ti-6Al-4V substrate was achieved in graded composition by combustion synthesis in high-gravity field. XRD, FESEM and EDS results showed that TiC-TiB2 ceramic coating was composed of fine TiB2 platelets, TiC irregular grains, Cr metallic phase and a few Al2O3 inclusions. The mechanical properties showed that the relative density, microhardness and fracture toughness of TiC-TiB2 ceramic coating reached 98.5%, 25.8 ± 3.5 GPa and 16.5 ± 2.5 MPa • m0.5, respectively. The fusion-bonding area and heat-affected zone of Ti alloy were obviously observed between the ceramic coating and Ti alloy substrate, and within the fusion bonding area a number of ultralfine TiB platelets and fine Ti-enriched carbides were embedded in the matrix of α’-Ti martensite, whereas at the heat-affected zone of Ti-6A1-4V substrate some C, B atoms were also detected. As a coupled result of fusion bonding and inter-diffusion between liquid ceramic film and molten Ti alloy substrate, followed by TiB2-Ti peritectic reaction and subsequent eutectic reaction in TiC-TiB-Ti ternary system, the ceramic coating of TiC-TiB2 on Ti-6Al4V substrate was achieved in continuously-graded composition, presenting the transitional change in microhardness from the ceramic coating to Ti alloy substrate with high shear strength of 450 ± 35 MPa between the coating and the substrate.

By adjusting the mole ratio of C and B elements in combustion system, solidified TiC-TiB2 composites with different TiB2 mole fraction were achieved by combustion synthesis in high-gravity field. XRD, FESEM and EDS results showed that with increasing TiB2 content, the matrix of TiC-TiB2 composite ceramics transformed a number of fine TiB2 platelets from the TiC spherical grains, and fine-grained even ultrafine-grained microstructures were achieved in solidified TiC-50mol% TiB2 due to eutectic growth under rapid solidification of the ceramic. Properties showed that relative density, Vickers hardness and flexural strength of TiC-50mol%TiB2 simultaneously reached the maximum values of 21.5 ± 1.5 GPa and 860 ± 35 MPa , whereas TiC-66.7mol%TiB2 presented the maximum fracture toughness of 13.5 ± 1.5 MPa • m0.5. FESEM fractography analyses of the ceramics exhibited a mixed mode of transcrystalline fracture of TiC spherical grains and intercrystalline fracture of TiB2 platelets, and the tendency of intercrystalline fracture was obviously enhanced with increasing TiB2 content to be 66.7 mol%, resulting in enhanced toughening mechanisms of crack deflection, crack-bridging and pull-out by fine TiB2 platelets, thus, the highest flexural strength was achieved in TiC-50mol%TiB2 due to the achievements of both fine-grained microstructures and high fracture toughness in the full-density solidified ceramics.

Based on preparing TiC-40mol%TiB2 composite by combustion synthesis under high gravity, the ballistic targets with different-thickness ceramic tiles were achieved by lateral, cover and back confinements of shrink-fit steel. Ballistic testing showed that as the thickness of ceramic tile was smaller than 12 mm, the penetration of the ceramic tile was mainly controlled by the combination of the initial-shock and steady-state penetration, resulting in poor ballistic performance of the ceramic. As the thickness of ceramic tile was larger than 12 mm, the penetration of the ceramic tile was controlled by the combination of steady-state and third-stage (projectile deceleration and erosion acceleration) penetration, resulting in improved ballistic performance of the ceramic.

Based on using combustion synthesis under high gravity to prepare TiC-TiB2 composite, the ceramic armour targets were achieved by the heat-shrunk lateral, cover and back confinements of steel sleeves and plates. Ballistic testing showed that under the impact of long-rod tungsten alloy projectile, the low-carbon-steel sleeve failed to laterally confine the ceramic due to its inadequate residual strength, resulting in poor ballistic performance of the ceramic, whereas the medium-carbon-steel sleeve had a sufficient residual strength to support lateral confinement of the ceramic as the long-rod tungsten alloy projectile makes room in the ceramic for its penetration, the confined ceramic could present its real ballistic performance, and the results of ballistic testing promised lateral confinement and cover confinement of high-strength metal would be beneficial to the improvement in ballistic performance of ceramic armour.

By using combustion synthesis under high gravity, TiC-TiB2 fine-grained composite ceramics with hypoeutectic, eutectic and hypereutectic microstructures were prepared through rapid solidification. XRD, FESEM and EDS results show that with increasing TiB2 content, TiC-TiB2 composite ceramics transform the microstructures consisting of fine TiB2 platelets from ones composed of fine TiC spherical grains, whereas when TiB2 content reaches 50mol%, the ceramics develop the eutectic microstructures that small aspect-ratio TiB2 platelets were embedded in TiC matrix. Mechanical properties show that relative density, Vickers hardness and flexural strength of TiC-50mol%TiB2 all reach the maximum values (respectively as 98.6 % ,18.4 GPa, 840 MPa) due to eutectic reaction during solidification, meanwhile TiC-50mol%TiB2 has the maximum fracture toughness of 11.5 MPa∙m0.5 due to the cooperative action of crack deflection, crack-bridging and pull-out toughening by fine TiB2 platelets.

Large-bulk TiB2-TiC composite ceramics were prepared by combustion synthesis under high gravity. XRD, SEM and EDS results showed TiB2-TiC composites were mainly composed of the fine-grained microstructures of TiC matrix in which a large number of the fine TiB2 platelet grains were dispersed uniformly, whereas there discontinuously dispersed the ε-carbides with the enrichment of Ti atoms, and a few of isolated, irregular α-Al2O3 grains and Al2O3-ZrO2 colonies were also observed at the boundaries of the eutectic microstructures. The results of properties indicate that with increasing mass fraction of B4C+Ti+C in combustion systems, the relative density and fracture toughness of TiB2-TiC composites are all among 97%~99% and 6.5~7.1 MPa·m1/2, respectively, and the Vickers hardness and flexural strength are increased gradually to the maximum values of 28.6GPa and 615MPa, respectively. The achievement of full-density TiB2-TiC composites benefited from the design of full-liquid SHS products and the introduction of high-gravity field, and high hardness of the composite ceramics resulted from the absence of intermediate borides and the achievement of stoichiometric TiC phases due to rapid solidification, whereas high flexural strength of the composite ceramics benefited from the homogenization and refinement of the microstructures due to the rapid separation of the liquid oxides and the rapid coupled growth of TiB2-TiC.

By adding ZrO2 and Y2O3 powder blend into the thermit, large bulk Al2O3/ZrO2 (Y2O3) eutectics were prepared by combustion synthesis under high gravity, and the influences of high gravity on microstructures, crystal growth and properties of the materials were also discussed. The XRD patterns showed that the introduction of high gravity field did not change phase constitution of the ceramics, and the ceramic matrix was mainly composed of α-Al2O3, t-ZrO2 and m-ZrO2. SEM images and EDS analyses showed that with increasing high gravity level, the morphologies of the ceramic microstructures transformed from the cellular eutectics to the rod-shaped colonies, and volume fraction and aspect ratio of the rod-shaped colonies increased while the rod-shaped colonies were refined; as the high-gravity field was larger than 200g, the microstructures of composite ceramics developed as the randomly-orientated rod-shaped colonies with a symmetrical triangular dispersion of tetragonal ZrO2 fibers of 300nm in the average diameter. Relative density, hardness, flexural strength and fracture toughness simultaneously reached the highest values of 98.6%, 18.6GPa, 1248MPa and 15.6MPa·m1/2 as the maximum high-gravity level of 250g was achieved. The increases of relative density and hardness of the ceramics with the high-gravity level are attributed to the acceleration of gas-escape from ceramic melts and the elimination of shrinkage cavity in the ceramics. The increase in fracture toughness results from the enhancement of the coupled toughening mechanisms while the increase in flexural strength comes from the refinement of the microstructures, the decrease of critical defect size and the achievement of high fracture toughness.

By introducing self-pressure processing into combustion synthesis under high gravity, Al2O3-ZrO2(4Y) eutectic composite ceramics without shrinkage cavities and holes were achieved, and the self-pressure processing also made the macro-crack be controlled and the ceramic densification be promoted evidently. XRD results showed the eutectic ceramics were composed of α-Al2O3, t-ZrO2 and a few m-ZrO2 phases, and the volume fraction of the transformable t-ZrO2 increased with self-pressure force increasing. SEM images showed that with increasing self-pressure force, the rod-shaped eutectic colonies were refined and the volume fraction of the colonies increased. Meanwhile, the irregular ZrO2 crystals around eutectic colonies transformed to be fine t-ZrO2 spherical crystals gradually, and thickness of eutectic colonies decreased. With the changes in microstructures of the ceramics, the mechanical properties of the ceramics were improved greatly.

Large bulk solidified TiC-TiB2 eutectic composite ceramics were prepared by combustion synthesis under high gravity, and the WO3 was introduced into the combustion synthesis as one of oxidants in thermit to obtain the Ti-W-Cr-C-B liquid, so near-full-density TiB2-(Ti,W)C eutectic composite ceramics without the macrocracks were achieved. The ceramic matrix was mainly composed of TiB2-(Ti,W)C eutectic microstructures, and a few of Al2O3 and Al2O3-ZrO2 eutectic structures were also detected in between Ti-W carbides. Due to the introduction of the high gravity field, Stocks immigration of the immiscible liquids took place due to their density differences, resulting in float-up of oxide liquid and settle-down of Ti-W-Cr-C-B liquid, and the layered melt consisting of oxide liquid and Ti-W-Cr-C-B liquid was formed, finally, TiB2-(Ti,W)C eutectic composite grown from the melt. Due to the mutual solubility of W-Ti, the W atom diffused into the TiC, leading to the formation of (Ti,W)C solid solution as same as crystal lattice structure of TiC. The relative density, Vickers hardness and fracture toughness of the composite ceramics measured 98.4%, 26.4 GPa and 7.6±0.5 MPa•m1/2, respectively.

The large bulk Al2O3/ZrO2 (Y2O3) eutectics were achieved by combustion synthesis in high-gravity field. With increasing high-gravity level, the matrix of eutectics transformed the rod-shaped colonies from the cellular ones, and the nanocrystalline microstructures came into existence as the high-gravity level was larger than 200g. The relative density, hardness, flexural strength and fracture toughness increased simultaneously with increasing high-gravity level, and reached the maximum values of 98.6%, 18.6GPa, 1248MPa and 15.6MPa•m1/2 respectively as the high-gravity level was 250g.